1. Field of the Invention
This invention relates to lightweight piston design, and more specifically to improved structures for a carbon--carbon composite piston.
2. Description of the Related Art
Carbon--carbon composite materials, as used herein, refers to a predominantly carbon matrix material reinforced with predominantly carbon fibers. The properties of these materials may be tailored to produce any desired mechanical and physical properties by preferred orientation of the continuous or staple fibers in the composite materials; and/or by the selection of additives; and/or by thermal treatment of the fibers and matrix before, during, or after fabrication. Carbon--carbon composite materials may be cast, molded, or machined. The surface or near-surface material can also be treated and/or coated with oxidation protection or sealing material, or with a catalytic materials such as nickel.
Carbon--carbon composite materials have been developed for aerospace applications for which high-temperature strength, light weight, and dimensional stability are desirable. Current aerospace application of carbon--carbon composite materials includes use as heat-shield material on advanced aerospace vehicles. Non-aerospace applications have been suggested due to the mechanical and physical advantages of carbon--carbon composite materials. Current non-aerospace applications include use in brakes and clutches for high-performance automobiles and in brakes for aircraft.
Internal combustion reciprocating engines and compressors used for aerospace, military, and transportation applications must be lightweight and capable of operating at elevated temperatures and pressures. Current state-of-the-art piston manufacture employs aluminum alloys and steel because pistons composed of these materials can withstand the relatively high temperatures and pressures associated with operation of an internal combustion engine or compressor. However, engine and compressor pistons manufactured of steel and/or aluminum alloys are significantly heavier than pistons made of carbon--carbon composite materials. In addition, aluminum alloy and steel pistons lose strength at elevated temperatures, i.e. operating temperatures above 600 degrees Fahrenheit (F.), while carbon--carbon composite pistons retain their strength under operating conditions which exceed 1200 degrees F.
The inherently high coefficient of thermal expansion of aluminum alloys also necessitates larger clearances between an aluminum alloy piston and an engine cylinder wall, to allow for expansion of the aluminum alloy piston during high temperature engine operation. In order to seal the gap between an aluminum alloy piston and engine cylinder wall, the piston must be fitted with a plurality of piston rings which seal the gap until the aluminum alloy piston has expanded sufficiently. However, at operating temperatures above 300 degrees F., the mechanical strength of aluminum alloy pistons decreases dramatically. This strength loss precludes locating the uppermost compression piston ring too close to the top, or crown, of the piston. Thus, a crevice between the piston crown and the uppermost compression ring is created. This crevice allows raw, unconsumed fuel to escape directly into the atmosphere, which contributes to atmospheric pollution and reduces fuel efficiency.
Another disadvantage associated with aluminum alloy pistons is the noise created as the undersized aluminum alloy pistons "rock" within the cylinder chamber.